F4 PHY 力爭

想3轉

Peter throws a ball of mass 0.5KG vertically upwards .The ball is initially at reat at a height 60 cm above the ground,It is then accelerated through a vertical dsitance of 80cm and reaches a speed of 8ms-1 before it leaves the hands of the boy

a)Find the magnitude of the average force acting on the bal⋯⋯l by Peter
b)How long does the ball travel in air before it returns to the ground
我計黎計去都計唔倒個答案呀
b)答案=1.76S

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你可唔可以翻譯呢句It is then accelerated through a vertical dsitance of 80cm and reaches a speed of 8ms-1 before it leaves the hands of the boy
究竟個波最高點係唔係(60+80)cm ??

想3轉

係個波最高點係(60+80)cm
但係個波係係60CM果個位先開始向上升
U=0

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原帖由 想3轉 於 09-12-29 07:29 PM 發表
係個波最高點係(60+80)cm
但係個波係係60CM果個位先開始向上升
U=0

U=0既話 個波又點會向上升?

●小舞

u=0 既話 個波點會向上升-,-?
唔通......

鏡花水月;

最高點並非60+80cm
60+80 只是離開手時的高度。

a) u是0, v是8, 找a
v^2 = u^2 + 2as
8^2 = 0 + 2a(0.8)
a = 40ms^-2

F=ma
F=0.5(40)
F=20N

b) 當ball reaches the ground, 因高度是140cm, displacement是向下140cm, so s=-1.4
s = ut + 1/2at^2
-(0.6+0.8) = 8t + 1/2(-10)t^2
-(1.4) = 8t - 5t^2
5t^2 - 8t - 1.4 =0
t=1.76 or t=-0.158 (rejected)
t=1.76s